Sonic Waveform Data Processing

ODP logging contractor: LDEO-BRG
Hole: 1173A
Leg: 190
Location: Nankai Trough (NW Pacific Ocean)
Latitude: 32° 14.6634' N
Longitude: 135° 1.5085' E
Logging date: June 5-6, 2000
Bottom felt: 4801.9 mbrf
Total penetration: 734.34 mbsf
Total core recovered: 658.23 m (89.6 %)

TOOL USED: DSI (Dipole Sonic Imager)

Recording mode: Monopole P&S and Upper Dipole (Pass 1) and Monopole P&S and Lower Dipole (Pass 2).

High-quality sonic log waveforms were recorded during Leg 190 from 65 to 360 mbsf in Hole 1173A. Raw P- and S- wave data form all 3 recorded modes were processed after the cruise to extract P- and S-wave slowness (inverse of velocity). The work focused on evaluating the high variability observed in the P-wave log in the upper interval of the hole and an inversion (low Vp and Vs) below 350 mbsf. In these intervals, sonic signals were weak and difficult to identify using standard shipboard processing methods.

A phase picking algorithm fully described by Kozak et al. (2001), was used, which is based on the Hilbert transform of time domain signals and typically uses 16 points per cycle to identify the arrival times of various wave modes. Real-time picking and semblance processing algorithms are often used for computation of P- and S-wave logs by commercial logging services (e.g. Kimball and Marzetta, 1984) but caution is advised if high-resolution results are desired or when enlarged hole conditions are present. In these cases, resolution may be lost be averaging across the receiver array or from noisy and unstable receivers at certain depths. Using the high-resolution phase picking method and selecting receivers for processing improves the slowness estimates significantly. In addition, frequency filtering, wave propagation modeling, stacking and other numerical techniques are used prior to the slowness picking to achieve the highest quality results.

The outcome of the reprocessing was successful, and extracted improved P-wave logs and S-wave values as low as 280 m/s in the upper interval of the hole and below 350 mbsf. Artifacts in the P-wave log due to mode switching and in the S-wave log due to flexural wave resonance were removed. Dispersion corrections were not required in these low-velocity sediments. The P-wave log shows systematically higher velocity than from core measurements due to porosity rebound. S-wave log velocities as low as 280 m/s were measured using the DSI in the interval above 100 mbsf and below 350 mbsf at Site 1173. The reprocessed logs should be used in place of the shipboard logs for future research studies.

For further information about the processing please contact:

David Goldberg

Phone: 845-365-8674

Fax: 845-365-3182

References

Goldberg, D., 2003. DATA REPORT: Reprocessing of wireline sonic logs in turbidites and hemipelagic sediments at ODP Ste 1173, Scientific Results, 190, Ocean Drilling Program, College Station, TX (in press.

Kimball, C.V., and T.L. Marzetta, 1984. Semblance processing of borehole acoustic array data. Geophysics, 49 (3), 274-281.

Kozak, M. P. Boonen, and D. Siefert, 2001. Phase velocity processing for acoustic logging-while-drilling full waveform data, Trans. Soc. Prof. Well-Log Analysts, 42nd Ann. Sympos., paper PP.